1. Field of the Invention
The present invention relates to thermoplastic polymer coated, electrically conductive wire particularly useful in automotive engine compartment applications. More specifically but not by way of limitation, the invention relates to an ultra-thin polyphenylene sulfide (PPS) coated copper wire capable of meeting SAE performance standards (e.g., J1678 specifications at 125xc2x0 C. and as high as 150xc2x0 C.) at a nominal coating thickness of 8 mils and 10 mils.
2. Description of Related Art
Temperature requirements for the insulation materials of wire and cable used under the hood of today""s high performance cars continue to increase. Thermoplastic polyvinyl chloride (PVC) used in high volume in automotive wiring provides chemical and flame resistance, insulation capability, and reasonable toughness, but suffers from higher temperature capability and environmental concern. The difficulties of disposal and recycle of PVC resin are recognized as serious problems today. Incineration results in formation of significant quantities of hydrogen chloride and heavy metal residues. Additionally, PVC is not compatible with other plastics used in manufacture of automobiles, which creates problems during recycling operations.
Today""s automotive wiring requires high temperature capability, good chemical and flame resistance, good electrical properties, good low temperature flexibility, and toughness. WO 93/08234 (Hausmann), published Apr. 29, 1993, provides a partially grafted, flexible thermoplastic blend based on polyester thermoplastic, epoxy group-containing ethylene copolymer, and ionomer of an acid group-containing ethylene copolymer with high and low temperature capability but poor flame resistance. Addition of halogen can provide flame resistance but introduces the environmental issue described above.
Crosslinked ethylene vinyl acetate copolymers (EVA) filled with hydrated fillers provide flame retardancy without halogen but modest mechanical toughness precludes thin automotive coatings for space savings in today""s tightly configured engine compartments. Crosslinking by either peroxide or irradiation adds cost. In the case of peroxide crosslinking, capital equipment such as a continuous vulcanization (CV) tube is required in contrast to thermoplastic systems.
Polyphenylene sulfide (PPS) is a high temperature, semicrystalline, engineering thermoplastic with excellent chemical resistance, high heat deflection temperature, good electrical insulation properties, and inherent flame resistance without halogen. Its poor flexibility can be seen in low impact strength and low elongation at break. Thus, PPS use has been limited in wire and cable applications that require high temperature capability, impact resistance, and flexibility, such as wiring under the hood of automobiles.
Toughened alloy compositions formed by melt blending certain ethylene copolymers, certain polymeric grafting agents which contain reactive groups selected from at least one of epoxides, isocyanates, aziridine, silanes, alkyl halides, alpha-halo ketones, alpha-halo aldehydes, or oxazoline, along with a polymer selected from polyethylene, polypropylene and copolymer thereof, poly(butene-1), poly-4-methylpent-1-ene, polystyrene and copolymers thereof, polyphenylene oxide, polyphenylene sulfide (PPS) and polysulfone, are taught under U.S. Pat. No. 4,871,810 (Saltman).
Higher modulus alloys compositions containing PPS are disclosed in U.S. Pat. Nos. 5,625,002 and 5,654,358 (Toray). Neither Saltman nor Toray address suitable polyphenylene sulfide alloy compositions for flexible, high temperature, flame retardant applications such as wire and cable.
Consequently, there is a need in the art for a flexible, tough thermoplastic composition with low and high temperature capability, good electricals, and flame retardancy, preferably without halogen, for use in wire and cable applications, particularly automotive, under-the-hood wiring.
The present invention provides an electrically conductive wire comprising a metal core and an outer electrically insulating, polymer coating; wherein the polymer coating comprises:
(a) from 40% to 90% by weight polyphenylene sulfide;
(b) from 9% to 50% by weight of a polymeric grafting agent, the grafting agent comprising a copolymer wherein for every one hundred parts by weight copolymer (i) at least 50 parts by weight are derived from ethylene comonomer; (ii) from 0.5 to 15 parts by weight are derived from one or more reactive comonomer selected from the group consisting of an unsaturated epoxide of 4 to 11 carbon atoms; an unsaturated isocyanate of 2 to 11 carbon atoms; an unsaturated alkoxy silane and/or unsaturated alkyl silane, wherein the alkoxy and the alkyl group is from 1 to 12 carbon atoms; and an unsaturated oxazoline; and (iii) from 0 to 49 parts by weight are derived an optional third comonomer selected from the group consisting of an alkyl acrylate, alkyl methacrylate, vinyl ether, carbon monoxide, and sulfur dioxide, where the alkyl and ether groups are of 1 to 12 carbon atoms; and
(c) from 1% to 20% by weight of an ethylene copolymer, wherein for every one hundred parts by weight ethylene copolymer (i) at least 50 parts by weight are derived from ethylene comonomer, (ii) from 1 to 35 parts by weight are derived from an acid-containing unsaturated carboxylic acid or anhydride comonomer, and (iii) from 0 to 49 parts by weight are derived from a comonomer selected from the group consisting of alkyl acrylate, alkyl methacrylate, vinyl ether, carbon monoxide, and sulfur dioxide, and further wherein the acid groups are neutralized from 0 to 100% by a metal ion,
wherein the wire meets SAE J1678 specifications at 125xc2x0 C. or higher at a thickness of the polymeric coating of less than 12 mils.
For purposes of the present invention the phrase xe2x80x9cpartially graftedxe2x80x9d means that the ethylene copolymer (c) is the limiting component so that the grafting reaction between the polymer grafting agent (b) and the ethylene copolymer (c) is limited by the amount of acid functionality of the ethylene copolymer, i.e. the use of more ethylene copolymer than is permitted by claims herein would result in an increased level of crosslinking between components which would adversely affect the beneficial properties of this invention.
In this disclosure, the word xe2x80x9ccopolymerxe2x80x9d means a polymer polymerized from two or more monomers, and includes terpolymers. The more specific description xe2x80x98ethylene acrylic acid copolymerxe2x80x99, xe2x80x98ethylene methacrylic acid copolymerxe2x80x99, and the like, is meant to include copolymers which may also have a third monomer present.
Ethylene/acid copolymers, i.e. xe2x80x9cacid copolymersxe2x80x9d and their corresponding ionomers are well known in the art to be copolymers of ethylene with an olefinically unsaturated organic mono- or di-acid such as acrylic or methacrylic acid, or maleic acid or fumaric acid or their anhydrides, the acid (or anhydride) comprising about 0.5 to 50 mole percent of the total polymeric material. The ethylene/acid copolymers and their methods of preparation are well known in the art and are disclosed in, for example, U.S. Pat. Nos. 3,264,272; 3,404,134; 3,355,319 and 4,321,337. The copolymers are termed ionomers when the acid is neutralized in whole or in part to produce a salt. The cations for the salts are usually an alkali metal such as sodium, potassium, zinc or the like. xe2x80x9cAcid copolymersxe2x80x9d or xe2x80x9cionomersxe2x80x9d referred to herein may be direct copolymers or graft copolymers.
Ethylene/acid/acrylate terpolymer and corresponding ionomers are well known in the art to be copolymers of ethylene, an olefinically unsaturated organic acid such as acrylic or methacrylic acid and an alkyl acrylate or methacrylate termonomer (e.g. n-butyl acrylate or methacrylate or isobutylacrylate).